There are many reasons, but one comes to mind because it's been happening recently: a banned user has repeadly tried to reregister. His ip is different, and he deletes his cookies, but I know it's him because of the unique ISP and geographic location.
When I contact an ISP they insist on detailed information.
Right now I need to copy all information before rejecting the registration. It would be great to be able to click a button and have access all the registration attempts.